Abstract: A variable volume ratio screw compressor has a side load inlet port for the injection of refrigerant vapor into the interlobe volume. In order to improve efficiency, overcompression and undercompression are avoided by varying the location of the radial discharge port to give the compressor an internal volume ratio matched to the pressure of the system in which the compressor operates. This is accomplished in the present application by locating a pressure sensing port no earlier than and preferably later in the compression than the side load injection port, but early enough in the compression that it will not communicate with the discharge port. The pressure that is sensed in the sensing port is used to predict the actual peak pressure in order that maximum efficiency may be obtained as a result of matching the internal volume ratio to the pressure ratio of the system.